An echo canceller is a device which generates a synthetic echo signal based on a first signal which gives rise to a real echo signal, and which combines the synthetic echo signal with a second signal that is degraded by the real echo signal in such a manner as to obtain a small a residual echo signal as possible. Canceller operation is based on the fact that a real echo path can be considered as a synthesizable linear filter. A canceller comprises an adaptive filter which generates the synthetic echo signal and which is adjusted to minimize the correlation between the originating signal which gives rise to the echo and the signal which is degraded by the residual echo.
Most telephone channel echo cancellers are digitally implemented and are designed to process signals which are available in the form of a series of digital samples supplied at a regular rate 1/T which is higher than the Nyquist rate for the signals. Their adaptive filters are constituted by non-recursive time domain transversal filters comprising a delay line having intermediate tapping points spaced at intervals equal to the sampling period T, and by processor means arranged during each sampling period T to weight the samples available at the tapping points, to sum the weighted samples, and to update the weighting coefficients.
The length of the delay line determines the maximum length possible for the adaptive filter's impulse response which must be substantially equal to the reverberation time of the echo to be cancelled. The reverberation time of an acoustic echo may easily be several hundreds of milliseconds long. Since the sampling period T is generally the 125 .mu.s period used by PCM encoding, the number of weighting coefficients that need to be used and updated turns out to be excessively large. The practical limit imposed by using processor means of reasonable power is about 250 weighting coefficients, which in a prior art echo canceller corresponds to an impulse response with a maximum duration of about 30 milliseconds, which is too short for dealing with acoustic echoes.
Preferred embodiments of the present invention increase the length of the maximum impulse response achievable for a given amount of calculation to be performed in one sampling period T.